Compound semiconductor single crystals, such as cadmium telluride (CdTe), cadmium zinc telluride (CdZnTe) and the like, have advantages that a light emission efficiency thereof is higher than that of silicon (Si) and that a heterojunction can be applied thereto. The compound semiconductor single crystals are expected to be applied to a light emitting element, a photodetector, a low-noise amplifier, or the like.
In particular, a CdTe crystal has a zinc blende structure, has a property that an energy gap thereof is 1.5 V and is a material which can have both a p-type conductivity and an n-type conductivity. Therefore, the CdTe crystal is used for semiconductor devices, such as a substrate for an epitaxial growth of an infrared detector, a solar battery, a visible light sensor, an infrared sensor, a radiation detector for .gamma.-ray or for X-ray, a non-destructive detector and an HgCdTe mixed crystal epitaxy (a far infrared detector), or the like.
Because many properties of these devices depend on a purity of a compound semiconductor single crystal for forming a substrate, it is desired to obtain a high purity compound semiconductor single crystal in order to improve performance of the devices.
Conventionally, a CdTe crystal or a CdZnTe crystal is produced by the Bridgman method (Bridgman Method), the gradient freezing method (Gradient Freezing Method: GF method), the vertical gradient freezing method (Vertical Gradient Freezing Method: VGF method) or the like. However, there is a problem that a grown crystal includes many deposits having Cd or Te, which cause the properties of these devices to be lower (for example, a deterioration in performance to detect infrared ray).
The method for reducing deposits having Cd or Te has been studied. For example, the reducing method is reported by H. R. VYDYANATH et al. Journal of Electronic Materials, Vol. 22, No 8, 1993 (p. 1073).
The above method for reducing deposits having Cd or Te was that a grown bulk CdTe crystal or CdZnTe crystal was cut in a wafer form to carry out a predetermined heat treatment.
However, when the above heat treatment was carried out to the crystal, there was a problem that a full-width-half-maximum (FWHM value) of a double crystal X-ray rocking curve became higher than that of the crystal to which the heat treatment was not carried out.
When the FWHM value of the double crystal X-ray rocking curve thereof was high, after the HgCdTe mixed crystal epitaxy, the FWHM value of the double crystal X-ray rocking curve of the surface of the grown HgCdTe mixed crystal became high. There was a problem that the property of a photodiode manufactured by using the epitaxy was deteriorated.
Further, there was a problem that an etch pit density (EPD) of the CdTe crystal or that of the CdZnTe crystal became high after the heat treatment.
That is, for example, an experience in which the heat treatment was carried out to the CdTe substrate or the CdZnTe substrate under Cd pressure atmosphere, at 850.degree. C. and for 20 hours was attempted. It was found that the EPD value which was 4.times.10.sup.4 cm.sup.-2 before the heat treatment, increased to 1.2.times.10.sup.5 cm.sup.-2 after the heat treatment.
The inventors studied about the above-described situations. As a result, the present invention was completed. An object of the present invention is to reduce the etch pit density (EPD) and the full-width-half-maximum (FWHM) value of the double crystal X-ray rocking curve, and to provide a CdTe crystal or a CdZnTe crystal which does not include deposits having Cd or Te and the process for producing the same.